The British government's human rights record since the attacks of 9/11 and the invasion of Iraq is facing ferocious criticism from a United Nations panel, which warns that prompt action is needed to ensure the country meets its obligations under international law.

In a report published on Friday, the UN Committee against Torture recommends more than 40 separate measures which it says will need to be taken if the UK is to be given a clean bill of health.

The report – which will doubtless make uncomfortable reading across Whitehall – contains the harshest criticism that the committee has yet made of a British government. It is the first substantial criticism since 1992, when the UK was told that were it not for the mistreatment of terrorism suspects in Northern Ireland, it would have been found to have "met in virtually every respect" its obligations under the UN convention against torture.

Its publication follows a two-day hearing in Geneva earlier this month, during which some committee members angrily accused the British government delegation of providing evasive answers to their questions about the UK's human rights record in recent years.

While the committee's report concentrates on human rights violations that predate the 2010 general election, it repeatedly expresses concern at the failure of the current government to take steps to hold those responsible to account.

Addressing the government's decision to suspend the judge-led inquiry into British involvement in torture and rendition since 9/11, the committee recommended that it "establish without further delay an inquiry on alleged acts of torture and other ill-treatment of detainees held overseas committed by or at the instigation of or with the consent or acquiescence of British officials". The perpetrators identified by this inquiry should be "duly prosecuted and punished appropriately" and the victims compensated, the report says.

The committee condemned what it described as an "escape clause" in the 1988 Criminal Justice Act, the piece of legislation that incorporated the UN torture convention into UK law. It called for the repeal of the clause, as it provides British officials with a defence against prosecution for torture if they can show that they had "lawful authority, justification or excuse" for inflicting severe pain or suffering.

The report says the committee is "deeply concerned at the growing number of serious allegations of torture and ill-treatment, including by means of complicity, as a result of the state party's military interventions in Iraq and Afghanistan".

The failure to secure convictions at court martial following the murder of Baha Mousa in Basra in 2003, and the one-year sentence imposed on the one soldier who admitted a criminal charge of inhumane treatment, was a matter for "deep concern", the committee said, as was the failure to prosecute anyone for the torture of other Iraqi prisoners.

"The committee regrets that the state party continue to resist a full public inquiry that would assess the extent of torture and ill-treatment and establish possible command responsibility for senior political and military figures," it said.

The report recommends that British soldiers and intelligence officers receive training that will help them to understand that the use of torture is absolutely prohibited in international law.

The committee also expressed concern that the secret court procedures introduced by the Justice and Security Act, which comes into force in July, could be used to deploy hearsay evidence or evidence obtained through torture.

It also said the new secrecy regime should not be permitted to conceal evidence of human rights violations – something the new law's critics believe to be its intention.

The report urges the government to:

• Rewrite its guidance to intelligence officers who are trying to obtain information from prisoners held by states with poor human rights records, as there is still a risk it could result in people being tortured.

• Cease reliance on "unreliable and ineffective" diplomatic assurances when seeking to deport people to countries where they risk being tortured.

• Ensure that police officers fire Taser weapons only when "there is a real and immediate threat to life or risk of serious injury".

The committee has given the British government 12 months in which to explain how it can make improvements in four key areas. These are the establishment of an inquiry into the UK's involvement in torture overseas; ensuring Sri Lankan asylum seekers are not forcibly returned when they face mistreatment at home; securing the release from Guantánamo of the British resident Shaker Aamer; and establishing inquiries into human rights violations committed during the Troubles in Northern Ireland.

Asked about the concerns raised by the committee, a spokesperson for the Ministry of Justice, which led the British delegation to Geneva earlier this month, issued a statement saying the British government "does not engage in torture, or solicit, encourage or condone its use" and works to prevent torture occurring.

The statement added that the government took seriously its responsibilities under the convention against torture: "The government is currently considering the recommendations made by the UN committee in its most recent report."